The generation of a concentration gradient is very important in many biological and chemical research processes, and analyte concentration is an important parameter in every chemical or biochemical reaction. Taking cytotoxicity assay as an example, a drug will exhibit efficacy at an optimal concentration but may be toxic at high concentrations. To evaluate the optimal dosage, serial dilution is a simple and common approach. Usually many individual experiments with descending concentrations are performed where the optimal concentration will lie within a concentration range. The more individual dilution experiments that are performed, the closer the range and the more accurate the result.
On-chip monitoring of cell reactions has great potential for automating such experiments and enabling large numbers of experiments to be performed easily and repeatedly. However, on-chip monitoring poses a number of technical challenges including the issues of cell manipulation and the generation of a concentration gradient.
Concerning cell manipulation, there have been a number of previous proposals for manipulating biological cells on a microfluidic device. These include electrokinetic transport techniques, entrapment of a cell by a constriction structure, the use of weirs, capturing cells by grid, negative dielectrophoresis (nDEP) traps, and cages for single eukaryotic cells.
To avoid mammalian cell distortion, gentle fluid manipulation is required, such as the technique of negative dielectrophoresis (nDEP). In nDEP, cells are levitated and contact-free in solution. Depending on the electrode array structure, funnel, aligner, cage and switch modules can be performed and controlled by AC current. However, efficient particle trapping requires very low flow rate (micro liters/hr). In addition, the fabrication and alignment of three-dimensional microelectrode systems are also complicated. Single cell manipulation is possible, however, where an embryonic cell was trapped by shallow constriction with less complicated channel geometry.
Dilution in micro-devices involves joining analyte and buffer streams followed by diffusive mixing. In some of the dilution models, rapid equilibrium is achieved by repeatedly split, mixed and recombined streams of fluorophore and buffer. Other devices examine concentration gradient in short straight channel before analytes were equilibrated within channel.